1. Field of the Invention
This invention relates to heat treatable hardfacings. In particular, this invention relates to a fabrication process for a powder composite core for subsequent use in a sheathing/compaction process for making a high-density powder composite rod for weld-applied hardfacing.
2. Description of the Related Art
Hard metal overlays are employed in rock drilling bits and other downhole tools to form wear and deformation resistant cutting edges and faying surfaces. These overlays comprise composite structures of hard particles in a metal matrix. Such hard metal overlays are normally formed by brazing or weld deposition of composite rod, producing a metal alloy matrix solidified from a melt containing hard particles that remain at least partially solid. Early examples of hardfacing rods for welding are shown in U.S. Pat. No. 1,757,601 and 3,023,130.
Hard metal overlays used on steel-toothed rolling cutter drill bits are subjected to extreme loads and prolonged scraping action. Therefore, the strongest, most wear resistant of fused hard metals are used in these cutting structures. Typically, such hard metal composites utilize sintered pellets or grains of cemented tungsten carbide/cobalt as the primary hard phase.
In addition to steel tooth rolling cutter drill bits, other types of down-hole tools also benefit from a strong wear resistant hardfacing material. For example, fixed cutter type earth boring drill bits and stabilizers often utilize welded hardfacing to protect gauge, blade, or watercourse surfaces. In a relatively recent development, tools made to steer drill bits during the drilling operation provide amongst the most demanding applications for hardfacing materials.
The formulation of composite rod filler metal entails fabrication and process considerations in addition to constituency selection. Typically, a tubular construction has been employed wherein a metal sheath is formed to enclose a particulate mixture comprising hard particles phases and additives including binders and de-oxidizers. In such a rod, the sheath metal combines with substrate melt, if any, to provide substantially all of the matrix phase of the final composite. The constituency of the hardmetal deposit is dependent on deposition process parameters as well as on the raw material formulation.
The management of thermal inputs during weld deposition is critical to deposit soundness and performance. Insufficient substrate heating and/or insufficient filler metal superheat can cause poor bonding, porosity, and irregular deposit configuration. Excess substrate heating, and/or excess filler metal superheat, and/or prolonged molten time produces substrate dilution and hard-particle degradation. Substrate dilution reduces carbide fractions, while sintered particle degradation causes softening and matrix embrittlement.
As the carbide loading of the rod and surface area of the application substrate increase, weld temperature and time control become increasingly critical. Composite rod with more than about 60% by weight carbide fill is problematic to weld without substrate penetration and dilution, especially on large substrates. Deposition dynamics are strongly influenced by the thermal transfer, melting, and flow characteristics of the composite rod.
Melting dynamics can be accelerated by incorporating the metal matrix components of the composite rod as a powder rather than as a solid sheath. This approach exploits the high specific surface area of the particulate material to speed up melting, while eliminating transport and mixing dynamics. However, sheath elimination also entails the loss of its structural contributions to handling strength and melt progression. U.S. Pat. Nos. 4,836,307; 4,944,774; and 5,051,112 (all incorporated by reference herein for all they disclose) disclose the sintering of a powdered composite rod as a means of replacing the mechanical strength of the sheath. Such sintered pre-forms develop a strong, porous structure which acts to impede heat flow prior to melt collapse into the weld pool. As a result, some melting speed is sacrificed and melt progression becomes more difficult to control, resulting in operator-induced thickness and composition variation in the hardfacing.
In U.S. Pat. No. 4,699,848, a wire-reinforced powder rod is disclosed, replacing external sheath with solid metal filler at the center of the rod, the location least easily melted by thermal flow from external heat sources. This construction exacerbates weldability and control limitations, compared with conventional practice.
In U.S. Pat. No. 5,740,872 (incorporated by reference herein for all it discloses) a powder composite rod is disclosed with a thin metal sheath wherein the ratio of powder metal to sheath metal exceeds 2.5. The fabrication of such a bound powder metallurgy composite rod for weld-deposited hard surfacing as described in this patent has been conducted by extrusion and curing of rod cores, followed by sheath attachment using a wrapping mill. The rod core produced in this process has a void volume of about 40 vol %, relying on the binder for green handling strength. The sheath is wrapped with a simple overlap and attached to the core by a silicate adhesive that partially infiltrates the porous core, providing additional handling strength and preventing core movement within the sheath. The silicate adhesive becomes a liquid slag during weld application that must be manipulated out of the deposit, slowing application rates and demanding greater operator skill. Difficulties associated with management of this slag lend to adverse deposit effects, including pellet degradation, porosity, inclusions, and reduced thickness control. Although the thin sheath extruded rod filler metal and application process provide net improvements in application productivity, quality, and in the hardfacing performance as compared with conventional practice, its utility is limited by silicate adhesive effects and also by the relative brittleness of low-density methylcellulose-bound powder cores.
As mentioned above, rod cores may be fabricated by extrusion into a trough or channel. The extruded material then is cured, such as by partial de-binding by desiccation. It has been found that during the extrusion and desiccation of rod cores, distortions can occur which include stretching, buckling, and sagging. The resulting cores exhibit bends and non-uniform section, causing difficulties in wrapping translating to finished rod variation that contributes to application variation.
An alternative fabrication method involves injection molding of cores into conventional split dies. This approach can substantially eliminate distortion but entails reduced productivity associated with prolonged core residence within the die necessary to achieve sufficient handling strength for removal.
The present invention is a method for fabricating a core for incorporation into a composite rod for hardfacing.
According to the present invention there is provided a fabrication process for a rod core comprising injecting rod core material into an elongate tubular mold, removing the mold to a curing station, and extracting the cured rod core from the mold.
By injecting the material into a closed rigid mold, rather than extruding it into a trough or channel, the buckling and stretching distortions are almost entirely eliminated. The use of a remote curing station allows material to be injected into a second mold while the material within the first mold is setting up, thus speeding up the manufacturing process.
The rod core material preferably comprises a powder mix of carbide and metal powders, a fugitive binder, and other additives. The mold may be of, for example, circular, oval, square, hexagonal or star-shaped cross-section.
The invention also relates to a method of fabrication of a hardfacing rod comprising encasing a rod core manufactured in accordance with the method defined hereinbefore in a metal sheath. The method may include a step of isostatically compacting the encased rod core to densify the core thereof and mechanically secure the sheath in position.
It is contemplated that the method has application to use hardfacing used in downhole tools including both fixed cutter and rolling cutter drill bits, bias pads for downhole rotary steerable systems, stabilizers, and other tools requiring strong and wear resistant hardfacings.